An email address identifies an email box to which email messages are delivered. A wide variety of formats were used in early email systems, but only a single format is used today, following the standards developed for Internet mail systems since the 1980s. An email address such as [email protected] is made up of a , an @ symbol, then a case-insensitive . Although the standard specifies the local part to be case-sensitive, in practice the mail system at example.com may treat John.Smith as equivalent to JohnSmith or even as johnsmith, and mail systems often limit their users' choice of name to a subset of the technically valid characters. In some cases they also limit which addresses it is possible to send mail to. With the introduction of internationalized domain names, efforts are progressing to permit non-ASCII characters in email addresses.

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